Previous approaches to loudspeaker design failed to consider the prospective interference effects between the two loudspeakers, one on the left and the other on the right, comprising stereo sound reproduction. These two combine to form a “loudspeaker system,” which also includes, but is not limited to, a quadraphonic or stereo system. Since the output of these two loudspeakers combine to produce a stereo image, interference is likely; they operate in parallel. To demonstrate this concept simply, two-way loudspeakers will be used in the stereo loudspeaker system. In addition to the interference and phase effects between the woofer and tweeter in either loudspeaker for the right or left channels, interference and phase effects are possible between the right tweeter and the left woofer as well as between the left tweeter and the right woofer. These concepts can be extended to sound reproduction in more than two channels like quadraphonic reproduction or home theater. Although the discussion of phase and interference in loudspeaker design can seem abstruse, these effects are quite audible.
Loudspeakers capable of reproduction approximating the entire audio band have been developed using various crossover circuitry and configurations. To extend the frequency response and power handling of a loudspeaker, multiple drivers are employed with each driver predominating in a specific portion of the frequency spectrum. Thus a loudspeaker can have woofers, tweeters and midranges, with tweeters reproducing higher frequencies, woofers reproducing lower frequencies and midranges reproducing the frequencies in between. A woofer, midwoofer, midrange, upper midrange or tweeter is called a “driver”. The typical two-way loudspeaker has a woofer or tweeter for drivers. Accordingly a 2.5-way loudspeaker is a modern design with a woofer, midwoofer and tweeter. Modern designs can use a midwoofer and a tweeter, but for the sake of simplicity, this will also be referred to as a woofer and a tweeter henceforward unless otherwise noted. A three-way loudspeaker has a woofer, midrange and tweeter. Each of the drivers is selected to perform best in a specific portion of the frequency spectrum, and a crossover circuit is applied to tailor driver response in this portion. The crossover network accomplishes this typically by attenuating driver response where undesired. The overwhelming majority of crossover networks connect the drivers in parallel and subsequent references to crossover networks refer to parallel circuits unless otherwise stated. The applicant will define the nouns “crossover network,” “crossover circuit,” or “crossover,” as referring to the network apportioning the different frequency bands of the input signal to the different drivers for the entire loudspeaker. The noun “filter” refers to the smaller network apportioning the given frequency band of the input signal to a single driver in the entire loudspeaker.
The frequency at which an audio crossover network delivers signals to two drivers operating in adjacent frequency ranges is called the crossover frequency. A crossover attenuates the response of a driver at the crossover frequency at a rate called the crossover slope. Crossover slopes are calculated in dB of attenuation per octave, with steeper slopes displaying more attenuation. The steepness of a crossover's slope is primarily determined by the number of capacitors and inductors used. For instance, passive crossovers in two-way loudspeakers having crossover slopes of 6 dB/octave generally have one inductor L or capacitor C for each filter in the crossover. These filters together form a 1st order electrical crossover. Crossover slopes of 12 dB/octave in two-way loudspeakers generally have one L and one C for each filter in the crossover, to total two inductors and two capacitors in the crossover. These two filters together form a 2nd order electrical, or half-section, crossover network. Analogously 4th order electrical crossover circuits are called full-section crossovers. These crossovers possess crossover slopes of 24 dB/octave and in two-way loudspeakers, generally have two inductors and two capacitors for each filter in the crossover, to total four inductors and four capacitors.
Loudspeaker drivers nonetheless reproduce waves and simultaneous reproduction from more than one driver at a given frequency produces interference effects. When two drivers of different size and shape are mounted on a conventional planar baffle, the depths of these drivers differ so that the fronts of these drivers' voice coils lie in different planes. For instance, a tweeter is typically smaller than a woofer and a tweeter cone is typically significantly shallower than a woofer cone. Accordingly when a tweeter and woofer reproduce the same frequency, the distances of the corresponding sound waves to the listener's ear differ, inducing interference. A crossover reduces these interference effects, but introduces its own interference effects. A crossover circuit between a woofer and a tweeter rolls the woofer response off at the crossover frequency, but gradually increases the tweeter response as the crossover frequency is approached. The woofer and tweeter responses at the crossover frequency are therefore out-of-phase to some extent. The crossed-over woofer and tweeter responses overlap substantially at some frequencies, where these responses are also out-of-phase to some extent.
Interference effects sound unpleasant. The original crossovers described in U.S. Pat. No. 3,457,370 to Boner were 2nd order electrical and accordingly introduced anomalies in frequency response whether the drivers were connected in-phase or out-of-phase, a deficiency characteristic of even-order electrical crossovers. Many listeners feel out-of-phase 2nd order electrical crossovers reproduce the human voice with a nasal quality. Accordingly he introduced impedance-correction networks into these crossovers.
Many other techniques have been proposed to improve the frequency response and phase behavior of loudspeakers. The interference effects between multiple drivers can be conveyed as a pair of drivers operating in-phase or out-of-phase. The more drivers there are in a given loudspeaker, the more possible driver pairs exist and consequently the more out-of-phase responses are possible. An example of a loudspeaker configuration diminishing undesirable phase effects is the d'Appolito configuration in which a specific driver configuration on the mounting baffle combined with a specific crossover type are applied. Polar response figures reveal the benefits of the popular d'Appolito configuration. There is nevertheless some variety among driver and crossover configurations yielding the characteristic d'Appolito phase behavior. Alternatively a loudspeaker can be configured with a stepped baffle so that the drivers are time-aligned. This configuration often reproduces more three-dimensional stereo images than conventional configurations.
Another approach to decrease loudspeaker interference effects in theory is to augment a loudspeaker with at least one auxiliary driver to improve the transfer function of the loudspeaker. The transfer functions of a woofer and a tweeter for a given crossover order differ so that the loudspeaker transfer function lacks fidelity with respect to the input for all but 1st order electrical crossovers. When an auxiliary driver is added with the appropriate crossover slopes, the fidelity of the loudspeaker transfer function is restored. Higher crossover orders entail more auxiliary drivers and more sophisticated selection of crossover slopes. At least one auxiliary driver is required for every crossover frequency, which divulges the problems generated with this approach. First the auxiliary driver will interfere with both the woofer and the tweeter in a two-way loudspeaker, which already interfere with each other in an unaugmented two-way loudspeaker. A crossover network tailors this interference, but does not eliminate it entirely. More drivers in an unaugmented loudspeaker simply produce more possible interference effects. Augmenting these loudspeakers with auxiliary drivers in the recommended approach simply compounds the possible interference effects. Moreover this approach corrects the transfer functions of the crossover network rather than those of the network plus the drivers. Drivers without a filter applied nevertheless roll off frequencies with characteristic slopes. The typical woofer rolls off high frequencies at approximately 12 dB/octave and the typical tweeter reaches full output at approximately 6 dB/octave from resonance. These characteristics are used in the determination of “effective” crossover orders, which refer to the slope of the roll off in frequency response that a driver filtered by a crossover actually displays. This is distinguished from the slope of the electrical filter in a crossover. Effective crossover orders complicate the recommended approach to loudspeaker design and provide transfer functions corresponding to the woofer and tweeter in a two-way system that differ even more. Thus design of the appropriate filter for the auxiliary driver is made more difficult, often enjoining the use of active crossover networks. Active crossovers can be used to optimize transfer functions, but like the approach using auxiliary drivers, are developed in the absence of actual drivers and their impedances, which depend on frequency.
Approximately infinite crossover slopes also render the auxiliary approach more difficult. These crossovers typically apply many sequential crossover sections to each driver in a loudspeaker. Accordingly many auxiliary drivers would be demanded for each pair of drivers consecutive in frequency. However some consider designing loudspeakers with approximately infinite crossover slopes sufficient improvement. Interference between a pair of drivers consecutive in frequency is reduced because there is little overlap in their frequency response. These loudspeakers can be enhanced by coupling adjacent inductors to increase slopes at diminished cost though the sheer number of crossover elements in these systems can be considered expensive. Furthermore active crossovers can be used, but at even greater expense.
The aforementioned loudspeaker designs connect the drivers in parallel. Drivers in a loudspeaker can be connected in series to minimize some interference and phase effects. Possible deficiencies of loudspeakers with series crossovers are limited selection in crossover slope, reduced efficiency and fewer possible designs. Loudspeakers with series crossovers often demand drivers with similar impedances. A transformer can be incorporated into series crossover networks to increase slopes to at least 2nd order. Recently 2nd, 3rd and 4th order series topologies have been developed using traditional crossover elements.
Most, if not all, the aforementioned crossover circuits and approaches can include impedance-compensation networks to smooth impedance and improve phase behavior. These networks can be applied across individual drivers as appropriate or across an entire loudspeaker.
The present art reduces phase and interference effects in sound reproduction and moderates lobing error between the loudspeakers comprising a loudspeaker system. The vertical polar response of a loudspeaker reveals lobe structure. Loudspeakers reproduce a spectrum of frequencies and lobe structure strongly depends on frequency. An increase in crossover order decreases driver overlap and thus lobing error, henceforth abbreviated as “lobing”. Lobing nonetheless remains at high crossover orders. Moreover the lobe structures of the loudspeakers comprising a loudspeaker system interact.
The art of the present invention applies to the prior art of paired loudspeakers using crossover circuits. It is an object of the present invention to reduce phase distortion and reduce interference effects compared to prior art crossovers, including the popular 1st order electrical crossover.
Another object of the present invention is to incorporate the concept of symmetry complemented by asymmetry for effective crossover orders in a pair of stereo loudspeakers to reduce phase distortion without significantly increasing cost.
A further object of the present invention is to incorporate the concept of handedness to distinguish effective odd-numbered crossover orders from effective even-numbered crossover orders and from prior art. This concept is also used in conjunction with specified polarity.